1. Field of the Invention
The present invention generally relates to fuel cell systems and, more particularly, to fuel cell systems with recirculation of a fluid stream.
2. Description of the Related Art
Electrochemical fuel cell assemblies convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cell assemblies generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cell assemblies typically employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between two electrode layers. The membrane, in addition to being an ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (i.e., fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or for circulating coolant. The plates are typically known as flow field plates. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are commonly referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
The fuel stream that is supplied to the anode typically comprises hydrogen. For example, the fuel stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. The oxidant stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air.
Each of the fuel cells making up a stack is typically flooded with the selected fuel and oxidant at a desired pressure. In certain systems, the desired pressure is kept constant regardless of load demand, while in other systems the desired pressure varies according to load demand. In all systems however, the desired pressure is generally controlled by a regulator at the source of the reactant. Such regulators can take many forms. For example, where the reactant originates from a source where gas pressure is higher than the desired pressure, the regulator can take the form of a variable opening valve system, which lets in as little or as much flow as necessary to maintain/attain the desired pressure: such regulators are typically called pressure regulators. In another example, where the reactant originates from a source where gas pressure is lower than the desired pressure, the regulator can take the form of a compressor. In yet another example, where the reactant originates from a source where gas pressure is substantially the same as the desired pressure, the regulator can take the form of a blower. More than one form of regulator can exist in a system. For example, Merritt et al., U.S. Pat. No. 5,441,821 discloses a system where the fuel originates from a high pressure source (pressurized hydrogen) and is controlled by a pressure regulator before reaching the stack, while the oxidant originates from a low pressure source (the environment) and is controlled by an air compressor (i.e., the air is compressed before reaching the stack).
Pressure regulators operate on a differential basis as the desired pressure is always set in relation to another pressure, which can be constant (e.g., maintain/attain a desired pressure over atmospheric pressure) or variable (e.g., maintain/attain a desired pressure over the pressure in some other part of the system, such other pressure being variable). For example, in the system disclosed by Merritt et al., U.S. Pat. No. 5,441,821, the compressor sets the desired oxidant pressure in relation to a constant pressure (atmospheric pressure), while the valve-type pressure regulator sets the desired fuel stream pressure according to a variable pressure, more specifically to maintain/attain a desired steady-state pressure differential between the fuel and oxidant streams.
Each reactant stream exiting the fuel cell stack generally contains useful reactant products, such as water and unconsumed fuel or oxidant, which can be made use of by the fuel cell system. On way to make use of such useful reactant products is to recirculate the exhaust reactant streams. Therefore, for example, recirculating the hydrogen exhaust stream to the anode inlet leads to a more efficient system as it minimizes waste that would result from venting the unconsumed hydrogen to the atmosphere.
As outlined in Merritt et al., U.S. Pat. No. 5,441,821, one way to effect hydrogen recirculation is through the use of a jet ejector, where the ejector's motive inlet is fluidly connected to the pressurized hydrogen supply, the ejector's suction inlet is fluidly connected to the hydrogen exhaust outlet and the ejector's discharge outlet is fluidly connected to the fuel cell stack's hydrogen stream inlet. As a result, according to the well known operation of jet ejectors, the hydrogen supply stream entrains (and therefore recirculates) the relatively low pressure hydrogen exhaust stream, with the two streams mixing before entering the fuel cell stack's anode inlet.
In light of the wide spectrum of fuel cell stack hydrogen inlet stream flow rates over which the jet ejector must operate, it has proven difficult to design a satisfactory jet ejector. Designing a jet ejector to supply the needed inlet flow rate to the fuel cell stack during maximum-load demand periods results in the nozzle and/or the throat portion of the diffuser being too large to recirculate the needed hydrogen during low-load demand periods (e.g., idle periods). Conversely, designing a jet ejector to recirculate the needed hydrogen during low-load demand periods results in the nozzle and/or the throat portion of the diffuser being too small to supply the needed inlet flow rate to the fuel cell stack during maximum-load demand periods.
To address the foregoing problem, a two-stage changeover ejector system has been proposed by Tatsuya et al., Japan Publ. No. 2001-266922, where one of either a low-flow or high-flow ejector is used depending on the conditions prevailing at the time. However, having two separate ejectors and the related fluid circuitry leads to space requirement concerns in typical automotive applications. Furthermore, the transition point in such systems, more specifically when the motive flow course is changed from the low-flow to the high-flow ejector, typically experiences a sudden drop in recirculation that often falls below the needed minimum entrainement level. Adding further ejectors would alleviate the transition point concern but, in turn, would aggravate the space requirement concerns.
There is therefore a need for a fuel cell system, with recirculation of a fluid stream, that can operate efficiently over the whole range of a fuel cell stack's operating conditions and that addresses some of the space requirement concerns typical in vehicular applications. The present invention addresses these and other needs, and provides further related advantages.